A neutral M²⁺-rich and M³⁺-poor (M = metal) metallurgical waste drainage was used to test a metal removal method based on the precipitation of layered double hydroxide (LDH). The LDH precipitation was induced by adding a salt of Al³⁺ (trivalent metal missing in the drainage) and maintaining or restoring the pH to a circum-neutral value. The precipitates were characterized by chemical analysis, XRD, ESEM, HRTEM and XAS. The main parameter controlling the removal of metals and the type of precipitate appeared to be the pH. As a function of pH variation during the experiments, analyses of precipitates and solutions showed either the formation of poor crystalline LDH combined with very high removal of Zn, Ni and Pb (92–100%), more variable removal of Mn (46–98%) and less Cd (33–40%), or the formation of more crystalline LDH combined with lower removal of Zn (62%), Mn (43%), Ni (88%), Pb (64%) and especially Cd (1%). The different metal removal efficiency in the two cases is only indirectly due to the different LDH crystallinity, and it is clearly affected by the following factors: 1) the two pH steps of the method; 2) the direction of pH variation within each step. In particular, the highest removal of metals is obtained when the first pH step goes towards acidic conditions, as a consequence of Al salt addition, and precipitation of a quasi-amorphous hydrated hydroxysulfate of Al (probably a precursor of felsӧbányaite Al₄(SO₄)(OH)₁₀ · 4H₂O) occurs. This first acidic pH step removes little or no metals (just 0–3%) but it is essential so that the second pH step towards slightly alkaline conditions, as a consequence of NaOH addition, can be highly efficient in removing divalent metals as the quasi-amorphous hydrated hydroxysulfate of Al gradually turns into an LDH incorporating Zn, Mg and other metals. On the contrary, when both pH steps remain in the neutral-alkaline range, only LDH precipitation occurs and a lower metal removal is observed. These results encourage further investigations on the removal of metals by inducing LDH precipitation as a simple and effective method for the treatment of circum-neutral polluted drainages.

使用中性的富含M ²⁺和贫M ³⁺（M =金属）的冶金废料来测试基于层状双氢氧化物（LDH）沉淀的金属去除方法。通过添加Al ³⁺的盐诱导LDH沉淀（排水中缺少三价金属），并将pH维持或恢复到环境中性值。通过化学分析，XRD，ESEM，HRTEM和XAS对沉淀物进行表征。控制金属去除和沉淀物类型的主要参数似乎是pH。在实验过程中，作为pH值变化的函​​数，对沉淀物和溶液的分析表明，要么形成不良结晶的LDH，同时锌，镍和铅的去除率非常高（92-100％），锰的去除率却变化较大（46-98） ％）和更少的Cd（33–40％），或者形成更多结晶的LDH，同时去除锌（62％），锰（43％），镍（88％），铅（64％）尤其是镉的可能性更低（1％）。两种情况下不同的金属去除效率仅是由于LDH结晶度不同而间接造成的，并且显然受以下因素影响：1）该方法的两个pH步骤；2）每个步骤中pH值变化的方向。特别是，由于添加铝盐和铝的准无定形水合羟基硫酸盐（可能是费斯班南特铝的前体）的沉淀，当第一个pH值步骤进入酸性条件时，金属的去除率最高。_发生4（SO ₄）（OH）₁₀ ·4H ₂ O）。这第一个酸性pH步骤几乎没有或几乎没有去除金属（仅0–3％），但是这是必不可少的，因此，由于添加了NaOH，第二个pH值朝着弱碱性条件的步骤可以非常有效地去除二价金属。 Al的无定形水合羟基硫酸盐逐渐变成结合了Zn，Mg和其他金属的LDH。相反，当两个pH步骤均保持在中性碱性范围内时，仅发生LDH沉淀，并且观察到较低的金属去除率。这些结果鼓励了通过诱导LDH沉淀去除金属的进一步研究，LDH沉淀是处理环境中性污染排水的一种简单有效的方法。